Accumulating an intermediate supply of weft thread for insertion into the shed is known in the art, the main object being to ensure a continuous speed of unwinding of the weft from the weft supply package while still enabling the pick to be inserted into the shed in a discontinuous manner.
Various types of weft accumulator systems are known. These are described below.
Certain types of weft accumulators are known in which a free loop is formed in the weft yarn. Such a device is described, for example, in the Swiss pat. No. 409,816 in which the weft thread is blown into a free loop by means of an airstream. The next pick insertion then pulls the loop straight again. The main disadvantage of this system is that when the pick is inserted, the thread has to be pulled straight against the force of the airstream, and so has to overcome a fairly strong resistance.
Another very common type of weft accumulator uses a weft prewinder. In this system, the weft thread is wound temporarily on a prewinder drum, and is then taken from the drum when the next pick is inserted. It is well known that the resistance necessary to pull the thread free from the prewinder drum is fairly great, thus limiting the thread velocity during picking.
In yet other known types of weft accumulator system, the weft thread is deposed in a mainly zigzag pattern on a flat surface, in other words forming several free loops. Such a system is described, for example in the French pat. No. 590,477, in which the weft thread is placed on a moving belt. The most important disadvantage of this type of accumulator is that the loops offer a large resistance when they are drawn off the belt, since the belt cannot be perfectly smooth. A certain amount of roughness of the belt surface is necessary to prevent the loops slipping off or piling up together when they are laid on the belt. Other disadvantages include wear and tear of the parts, since a mechanical drive is required, and the accumulation of dust, since the system is necessarily open.
In another system, described in the French pat. No. 1,449,084, an intermediate supply is accumulated by placing the weft thread in a stretched condition in a perforated tube.